The Hyak Tunnel just past Snoqualmie Pass (if you do it before winter - when that becomes a nordic area). You can walk across the Columbia river on a newly renovated railroad trestle that is part of the Palouse to Cascades Trail. Before you cross the Columbia, you can also check out the Gingko Petrified Forest interpretive center - it some great exhibits about the Great Missoula Floods and petroglyphs that were moved up the the dam flooded the river bank. After you cross the Columbia at Vantage, stop off at the overlook (if it is open) you can climb up to a sculputral exhibition of running horses and get a great view of the river. In Spokane check out the River Front Park. Cool towns along the route include Roslyn (where Northern Exposure was filmed) and Ellensburg. Here is a link to a map centered on the Hyak Trailhead. https://maps.northwestportal.com/outdoors/#14.11/47.393442/-121.388042 This view has the railroad bridge, visitor center and the overlooks (one in each direction) https://maps.northwestportal.com/outdoors/#10.87/46.910251/-119.971950 And here is the map centered on the park in Spokane: https://maps.northwestportal.com/outdoors/#14.92/47.660817/-117.420713